Fibrillated cellulosic fibers are material made from cellulose fibers, where the individual microfibrils of a fiber have been partly or totally detached from each other.
Microfibrillated cellulose (MFC) (also known as nanocellulose) is a one kind of fibrillated cellulose. MFC is normally very thin (˜20 nm) and the length is often between 100 nm to 10 μm. However, the microfibrils may also be longer, for example between 10-100 μm.
Fibrillated cellulose can be produced in a number of different ways. It is possible to mechanically treat cellulosic fibers so that microfibrils are formed. It is also possible to produce fibrils from cellulose by the aid of different chemicals and/or enzymes which will break interfibrillar bonds or dissolve the fibers. One example of production of MFC is shown in WO2007091942 which describes production of MFC by the aid of refining in combination with addition of an enzyme.
Fibrillated cellulose can be used within many different fields. In the papermaking industry it can both be added to the surface of a paper or board or to the furnish. It has been shown that addition of fibrillated cellulose can increase the strength of a paper or board. When used in paper coating applications, it replaces the synthetic or natural binders such as starch. Since the fibrillated cellulose has a high thickening effect at low solid content, it can be used to as thickening agent, immobilizing agent, water retention aid, lubrication agent, dispersing agent and/or stabilizing agent. However, it can also be used in many other technical fields, such as the food industry, polymer or plastic industry, paint, ceramic, ink, composite industry (e.g. cement), rubber industry, cosmetic and pharmaceutical industry.
A dispersion comprising fibrillated cellulose, such as MFC, has the appearance of a highly viscous, shear-thinning transparent gel at low dry contents. Normally, a composition comprising fibrillated cellulose with a consistency of about 4% and higher is in the form of a very thick gel. Very fibrillated and fine material with high degree of polymerization might exhibit a gel like character at solid content about or less than 1 wt %. The gel has high viscosity making it very difficult to make it flow at low shear rates. This makes processing through pipes and pumps very difficult and thus also to distribute it to different end uses, for example to a surface of a paper or board substrate.
Thus, it is often undesirable to add a composition with low dry content during production of paper or board to the surface of a substrate since it takes a lot of energy to remove the added water, for example during drying of the substrate. Addition of a dispersion with low solid content and strongly shear thinning characteristics might also require a special coating unit in order to prevent too much penetration and unevenness during coating. Another reason to avoid unnecessary addition of water is to save transportation costs, water and environmental impact (carbon footprint).
One way to solve these problems is to dry the produced MFC before addition to a composition such as a for instance a pigment dispersion or to the paper machine wet end but this is a very energy consuming process and might have significant irreversible chemical and physical structural changes on the microfibrillated cellulose. Another problem is that wetting agent might be required to facilitate the re-dispersion of the dried MFC.
There is thus a need for a process for the production of a composition comprising fibrillated cellulose with high dry content in an improved way.